a. Field of the Invention
This invention relates to a method and apparatus for providing diagnostic features for an optical transceiver, in which the optical transceiver uses electronic dispersion compensation (EDC) in order to alleviate distortion of a signal caused by dispersion.
As data rates and link lengths increase in modern communications channels the impact of signal distortion effects, such as dispersion, become more apparent. These can prevent transmission at high data rates over long lengths of fibres.
In a multimode optical fibre different modes have different propagation velocities, which tends to disperse a pulse into adjacent pulses, thereby causing inter symbol interference (ISI). Pulse dispersion also occurs in single mode fibre, but to a lesser extent. Such effects will therefore tend to close an eye pattern and increase the measured bit error rate (BER) at the receiver.
High-speed optical communications links, for example links operating at a data rate of at least 5 Gbit/s, have tended to use single mode optical fibre together with high precision optical fibre connectors. This has been the case even when such links are operating over short distances in local area networks where links are typically of the order of 10 m to 100 m in length. While such high speed communications links provide reliable performance at a very low BER, for example 10-12, there is a need for comparable performance at greatly reduced cost, and in practice this requires the use of multimode transmission and cheaper connectors, and preferably also cheaper and potentially less stable sources of optical radiation.
For multi mode fibre the effect of dispersion is reduced in the electronic domain (i.e., after the photodetector) by schemes, which are called electronic dispersion compensation (EDC).
For data rates of 10 Gb/s using multimode fibre without electronic dispersion compensation permits transmission up to ˜80 m (with a 1310 nm laser source). Local area networks have a large installed fibre base with link lengths up to ˜300 m which have been used at lower data rate. It is desirable to avoid the cost of replacing these with single mode fibre to increase the possible data rate. Therefore an optical transceiver that will work over this installed fibre base is required.
Such compensators can use an equaliser circuit to compensate for intersymbol interference caused by dispersion. Such an equaliser circuit receives as an input the output from a photoreceiver including a photodetector circuit, and then generates from this at least two equaliser coefficients. A signal delay line also receives the output from the photodetector circuit. Tapped outputs from the delay line are each multiplied or otherwise combined with one of the equaliser coefficients, and then summed together to generate an equalised output signal.
Provided that the coefficients are carefully optimised, the signal quality can be significantly improved. For a given transmitter and fibre, electronic dispersion compensation can increase the achieved transmission distances by e.g. 50%. The invention proposed herein seeks to further increase the improvement up to a factor of 2.5.
A technical challenge of great importance for applications is automatic adaptation of the parameters of the electronic dispersion compensator to the link properties, because the optimum parameters depend on the properties of the fibre link and the transmitter, and manual optimization is not cost-effective. Particularly in systems with multi-mode fibres, the optimum parameters may also drift with time.
One common approach is to use a least mean squared (LMS) algorithm to generate the equaliser coefficients. Such equalisers use an iterative approach that aims to converge on the correct equaliser coefficients slowly over many hundreds or thousands of repeat calculations. Although, it may be possible to achieve higher performance at increased cost and complexity in the equaliser.
Electronic Dispersion Compensation can be used to allow 10 Gb/s serial transmission over 220 to 300 m of Fibre Distributed Data Interface (FDDI) grade multimode fibre.
In this application there can be large variation in different fibres' channel characteristics. In some rare cases channel characteristics could be so bad that EDC may not be able to correct for the signal impairments caused by a particular fibre.
An additional complexity is the temporal variability in a fibre's characteristics. Potentially over time a fibre's characteristics could drift in and out of a state that could or could not be corrected for by EDC. This could result in a system that works when installed but where over time a link failure will occur.
This invention offers a simple method of diagnosing whether a fibre used in a link is likely to be corrected by the EDC, hence, giving an early indication of the likelihood of a link failure. This allows a system installer to avoid use of such fibres. The invention provides a dynamic indication of performance over a fibre, thus giving an indication of variation of the performance over time.
The IEEE standard (802.3aq 10GBASE-LRM) for a 10 Gb/s multimode fibre application permits two launches. An offset launch or a centre launch can be used. The intent of this is to widen fibre coverage as for many fibres dispersion can be bad at centre launch but good at offset launch or vice versa. The method of this invention seeks to provide a simple way of determining which launch may be best to use.